diff --git a/admin/src/views/exam/form.vue b/admin/src/views/exam/form.vue
index 8b33b555..d8eb1ab0 100644
--- a/admin/src/views/exam/form.vue
+++ b/admin/src/views/exam/form.vue
@@ -54,6 +54,12 @@
                             placeholder="Select End Time">
                         </el-date-picker>
                     </el-form-item>
+
+                    <el-form-item label="Duration" prop="duration">
+                        <el-input v-model="formData.duration" type="number" placeholder=""></el-input>
+                        <div style="color: #aaa">Unit: days. When assign to user, begin and end are used if they are specified. Otherwise begin time is the time at assignment, and the end time is the time at assignment plus the duration.</div>
+                    </el-form-item>
+
                     <el-form-item label="Target User Class" prop="filters.classes">
                         <el-checkbox-group v-model="formData.filters.classes">
                             <el-checkbox v-for="(item, index) in allClasses" :label="index" :key="index">{{item}}</el-checkbox>
@@ -109,6 +115,7 @@ export default {
                 description: '',
                 begin: '',
                 end: '',
+                duration: '',
                 indexes: [],
                 filters: {
                     classes: [],
@@ -141,6 +148,7 @@ export default {
                     state.formData.description = res.data.description
                     state.formData.begin = res.data.begin
                     state.formData.end = res.data.end
+                    state.formData.duration = res.data.duration
                     state.formData.indexes = res.data.indexes
                     state.formData.filters = res.data.filters
                     state.formData.status = res.data.status
diff --git a/admin/src/views/exam/index.vue b/admin/src/views/exam/index.vue
index 87b01a3f..a6e9abf8 100644
--- a/admin/src/views/exam/index.vue
+++ b/admin/src/views/exam/index.vue
@@ -50,6 +50,11 @@
             >
             </el-table-column>
 
+            <el-table-column
+                prop="duration_text"
+                label="Duration"
+            ></el-table-column>
+
             <el-table-column
                 label="Target users"
                 width="350px"
diff --git a/app/Console/Commands/Test.php b/app/Console/Commands/Test.php
index 04c57077..919707b5 100644
--- a/app/Console/Commands/Test.php
+++ b/app/Console/Commands/Test.php
@@ -7,6 +7,7 @@ use App\Models\ExamProgress;
 use App\Models\ExamUser;
 use App\Models\User;
 use App\Repositories\ExamRepository;
+use Carbon\Carbon;
 use Illuminate\Console\Command;
 use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Hash;
@@ -57,6 +58,7 @@ class Test extends Command
 //        $rep->cronjobCheckout();
         $r = DB::select(DB::raw('select version() as info'))[0]->info;
         dd($r);
+        Carbon::now()->toDateTimeString();
     }
 
 }
diff --git a/app/Http/Controllers/ExamController.php b/app/Http/Controllers/ExamController.php
index 974bbbf2..cb291fbc 100644
--- a/app/Http/Controllers/ExamController.php
+++ b/app/Http/Controllers/ExamController.php
@@ -44,6 +44,7 @@ class ExamController extends Controller
             'name' => 'required|string',
             'indexes' => 'required|array|min:1',
             'status' => 'required|in:0,1',
+            'duration' => 'numeric'
         ];
         $request->validate($rules);
         $result = $this->repository->store($request->all());
diff --git a/app/Http/Resources/ExamResource.php b/app/Http/Resources/ExamResource.php
index 9ed278ac..18263afb 100644
--- a/app/Http/Resources/ExamResource.php
+++ b/app/Http/Resources/ExamResource.php
@@ -23,6 +23,8 @@ class ExamResource extends JsonResource
             'description' => $this->description,
             'begin' => $this->begin,
             'end' => $this->end,
+            'duration' => $this->duration,
+            'duration_text' => $this->duration_text,
             'filters' => $this->filters,
             'filters_formatted' => $this->formatFilters($this->resource),
             'indexes' => $this->indexes,
diff --git a/app/Models/Exam.php b/app/Models/Exam.php
index c520abee..b8d307c1 100644
--- a/app/Models/Exam.php
+++ b/app/Models/Exam.php
@@ -4,7 +4,7 @@ namespace App\Models;
 
 class Exam extends NexusModel
 {
-    protected $fillable = ['name', 'description', 'begin', 'end', 'status', 'is_discovered', 'filters', 'indexes'];
+    protected $fillable = ['name', 'description', 'begin', 'end', 'duration', 'status', 'is_discovered', 'filters', 'indexes'];
 
     public $timestamps = true;
 
@@ -59,4 +59,9 @@ class Exam extends NexusModel
         return self::$discovers[$this->is_discovered]['text'] ?? '';
     }
 
+    public function getDurationTextAttribute(): string
+    {
+        return $this->duration . ' Days';
+    }
+
 }
diff --git a/app/Models/ExamUser.php b/app/Models/ExamUser.php
index 031ee86b..31bacdcb 100644
--- a/app/Models/ExamUser.php
+++ b/app/Models/ExamUser.php
@@ -24,19 +24,58 @@ class ExamUser extends NexusModel
         'progress' => 'json'
     ];
 
-    public function getStatusTextAttribute()
+    public function getStatusTextAttribute(): string
     {
         return self::$status[$this->status]['text'] ?? '';
     }
 
     public function getBeginAttribute()
     {
-        return $this->begin ?? $this->exam->begin;
+        $begin = $this->getRawOriginal('begin');
+        $end = $this->getRawOriginal('end');
+        if ($begin && $end) {
+            do_log(sprintf('examUser: %s, begin from self', $this->id));
+            return $begin;
+        }
+
+        $exam = $this->exam;
+        $begin = $exam->getRawOriginal('begin');
+        $end = $exam->getRawOriginal('end');
+        if ($begin && $end) {
+            do_log(sprintf('examUser: %s, begin from exam: %s', $this->id, $exam->id));
+            return $begin;
+        }
+
+        if ($exam->duration > 0) {
+            do_log(sprintf('examUser: %s, begin from self created_at', $this->id));
+            return $this->created_at->toDateTimeString();
+        }
+        return null;
     }
 
     public function getEndAttribute()
     {
-        return $this->end ?? $this->exam->end;
+        $begin = $this->getRawOriginal('begin');
+        $end = $this->getRawOriginal('end');
+        if ($begin && $end) {
+            do_log(sprintf('examUser: %s, end from self', $this->id));
+            return $end;
+        }
+
+        $exam = $this->exam;
+        $begin = $exam->getRawOriginal('begin');
+        $end = $exam->getRawOriginal('end');
+        if ($begin && $end) {
+            do_log(sprintf('examUser: %s, end from exam: %s', $this->id, $exam->id));
+            return $end;
+        }
+
+        $duration = $exam->duration;
+        if ($duration > 0) {
+            do_log(sprintf('examUser: %s, end from self created_at + exam: %s %s days', $this->id, $exam->id, $duration));
+            return $this->created_at->addDays($duration)->toDateTimeString();
+        }
+        return null;
     }
 
 
diff --git a/app/Repositories/ExamRepository.php b/app/Repositories/ExamRepository.php
index 5415ef24..8cf616ef 100644
--- a/app/Repositories/ExamRepository.php
+++ b/app/Repositories/ExamRepository.php
@@ -103,9 +103,8 @@ class ExamRepository extends BaseRepository
     {
         $now = Carbon::now();
         $query = Exam::query()
-            ->where('begin', '<=', $now)
-            ->where('end', '>=', $now)
             ->where('status', Exam::STATUS_ENABLED)
+            ->whereRaw("if(begin is not null and end is not null, begin <= '$now' and end >= '$now', duration > 0)")
             ->orderBy('id', 'desc');
         if (!is_null($excludeId)) {
             $query->whereNotIn('id', Arr::wrap($excludeId));
@@ -130,7 +129,6 @@ class ExamRepository extends BaseRepository
             do_log("$logPrefix, no valid exam.");
             return $exams;
         }
-
         $matched = $exams->filter(function (Exam $exam) use ($uid, $logPrefix) {
             return $this->isExamMatchUser($exam, $uid);
         });
@@ -253,8 +251,12 @@ class ExamRepository extends BaseRepository
         if (!$exam) {
             throw new NexusException("exam: {$examUser->exam_id} not exists.");
         }
-        $begin = $examUser->begin ?? $exam->begin;
-        $end = $examUser->end ?? $exam->end;
+        $begin = $examUser->begin;
+        $end = $examUser->end;
+        if (!$begin || !$end) {
+            do_log(sprintf("no begin or end, examUser: %s", $examUser->toJson()));
+            return false;
+        }
         if ($now < $begin || $now > $end) {
             do_log(sprintf("now: %s, not in exam time range: %s ~ %s", $now, $begin, $end));
             return false;
@@ -331,10 +333,6 @@ class ExamRepository extends BaseRepository
         }
         $examUser = $examUsers->first();
         $exam = $examUser->exam;
-        if (empty($examUser->begin) || empty($examUser->end)) {
-            $examUser->begin = $exam->begin;
-            $examUser->end = $exam->end;
-        }
         $progress = $this->calculateProgress($examUser);
         do_log("$logPrefix, progress: " . nexus_json_encode($progress));
         $examUser->progress = $progress;
@@ -344,25 +342,14 @@ class ExamRepository extends BaseRepository
 
     private function calculateProgress(ExamUser $examUser)
     {
-        $exam = $examUser->exam;
         $logPrefix = "examUser: " . $examUser->id;
-        if ($examUser->begin) {
-            $logPrefix .= ", begin from examUser: " . $examUser->id;
-            $begin = $examUser->begin;
-        } elseif ($exam->begin) {
-            $logPrefix .= ", begin from exam: " . $exam->id;
-            $begin = $exam->begin;
-        } else {
+        $begin = $examUser->begin;
+        $end = $examUser->end;
+        if (!$begin) {
             do_log("$logPrefix, no begin");
             return null;
         }
-        if ($examUser->end) {
-            $logPrefix .= ", end from examUser: " . $examUser->id;
-            $end = $examUser->end;
-        } elseif ($exam->end) {
-            $logPrefix .= ", end from exam: " . $exam->id;
-            $end = $exam->end;
-        } else {
+        if (!$end) {
             do_log("$logPrefix, no end");
             return null;
         }
@@ -507,7 +494,8 @@ class ExamRepository extends BaseRepository
             ->with(['exam', 'user', 'user.language'])
             ->orderBy("$examUserTable.id", "asc");
         if (!$ignoreTimeRange) {
-            $baseQuery->whereRaw("if($examUserTable.end is not null, $examUserTable.end < '$now', $examTable.end < '$now')");
+//            $baseQuery->whereRaw("if($examUserTable.end is not null, $examUserTable.end < '$now', $examTable.end < '$now')");
+            $baseQuery->whereRaw("case when $examUserTable.end is not null then $examUserTable.end < '$now' case when $examTable.end is not null then $examTable.end < '$now' case when $examTable.duration > 0 then date_add($examUserTable.created_at, interval $examTable.duration day) < '$now' else true end");
         }
 
         $size = 100;
diff --git a/database/migrations/2021_04_19_061650_create_exams_table.php b/database/migrations/2021_04_19_061650_create_exams_table.php
index 17fab1cb..09ae6026 100644
--- a/database/migrations/2021_04_19_061650_create_exams_table.php
+++ b/database/migrations/2021_04_19_061650_create_exams_table.php
@@ -19,6 +19,7 @@ class CreateExamsTable extends Migration
             $table->text('description')->nullable();
             $table->dateTime('begin')->nullable();
             $table->dateTime('end')->nullable();
+            $table->integer('duration')->default(0);
             $table->text('filters')->nullable();
             $table->text('indexes');
             $table->tinyInteger('status')->default(0);